The invention relates to a method for purifying mixtures comprising 4,4′-methylenediphenyl diisocyanate, which comprises purifying by distillation a mixture I comprising 4,4′-methylenediphenyl diisocyanate having a hydrolyzable chlorine content as specified in ASTM D4663-10 of greater than 100 ppm by means of a column K1, wherein the gaseous stream comprising the mixture I is brought into contact in the column K1 with at least one liquid compound A which has the same or higher boiling point than 4,4′-methylenediphenyl diisocyanate and which has a hydrolyzable chlorine content as specified in ASTM D4663-10 of a maximum of 100 ppm, and wherein the gaseous stream O obtained at the top of the column comprises 4,4′-methylenediphenyl diisocyanate has a hydrolyzable chlorine content as specified in ASTM D4663-10 of a maximum of 100 ppm.
Methylenediphenyl diisocyanate (MDI) is an important starting product for producing polyurethanes and related polymers that are used, for example, in foams and coatings. Pure 4,4′-MDI is a compound that is solid at room temperature and melts at 38° C.
The acid-catalyzed production of methylenediphenyl diisocyanate (MDI) starting from aniline and formaldehyde is known and leads first to a complex mixture of polyamines which is then reacted with phosgene. In this case, first a complex mixture of binuclear and polynuclear MDI is obtained which will hereinafter be called crude methylenediphenyl diisocyanate (crude MDI). Crude MDI comprises, in particular, the binuclear isomers 4,4′-MDI, 2,4′-MDI and to a lesser extent 2,2′-MDI (hereinafter termed together crude binuclear MDI) and also trinuclear or polynuclear MDI which will be termed hereinafter polymeric MDI (PMDI).
In known methods, crude MDI is separated into a PMDI-rich mixture and into crude binuclear MDI. Subsequently, customarily, 4,4′-MDI is separated off, firstly, and a 2,4′-MDI-rich mixture, secondly, is separated off from the crude binuclear MDI. Corresponding methods are described, for example, in the laid-open publications DE 1923214, DE 102005004170, DE 102005055189, CN 101003497 and DE 10333929.
For further use in said polymeric systems, in some cases a high purity, in particular a high isomeric purity, is necessary, since frequently only highly linear polymers of 4,4′-MDI have the desired end properties. In other cases, mixtures of the abovementioned isomers are used in the presence or absence of polynuclear MDI.
Before further processing, the methylenediphenyl diisocyanate products thus produced which are in liquid form must be stocked temporarily and/or stored.
Methylenediphenyl diisocyanate (MDI), in particular binuclear MDI, forms dimeric secondary products in the liquid phase after some time, i.e. during storage. In this case, in particular the formation of uretdiones by 4-membered ring formation owing to dimerization of two isocyanate groups and the formation of uretonimines by 4-membered ring formation from one carbodiimide group and one isocyanate group plays an important role. The formation of the 4-membered rings is in principle an equilibrium reaction which, by temperature elevation, can be shifted to the side of the isocyanates or carbodiimides. The formation of uretdiones also proceeds in the case of aromatic isocyanates uncatalyzed. A trimerization to give what are termed isocyanurates (1,3,5-triazine-2,4,6-triones) is likewise possible, but generally proceeds at a significant velocity only when a suitable catalyst is added.
The formation of the dimeric secondary products that are insoluble in the methylenediphenyl diisocyanate leads to disadvantageous hazes and sedimentations and results in quality decreases in the subsequent further processing, in particular owing to blockage of lines, apparatuses and machines.
A further problem is aromatic halogen compounds that are present in MDI. In the condensation of formaldehyde and aniline that is catalyzed by hydrochloric acid, chlorine-comprising byproducts form that are not at first separated off, but are reacted further with phosgene. In the reaction of the complex polyamine-comprising mixture with phosgene, further chlorine-comprising compounds form, in particular N,N-disubstituted (secondary) carbamoyl chlorides and chlorinated phenyl isocyanates.
Aromatic halogen compounds should be avoided, in particular, when, at elevated temperatures, they are chemically converted into compounds having readily hydrolyzable halogen. Hydrolyzable halogen compounds, in particular when they occur in variable concentrations, interfere with the reaction of isocyanates with polyols to form polyurethanes, since the reaction rate is effected by the halogen compounds. In addition, they cause a more rapid yellow discoloration of the isocyanates occurring first clear and colorless. From a multiplicity of such aromatic halogen compounds those which may be mentioned by way of example are: N,N-dimethylaniline hydrochloride, N-chloroformylaniline, N-methyl-N-chloroformylaniline and also compounds of the formulae

A method which reduces the content of aromatic halogen compounds in mixtures comprising 4,4′-MDI or provides such a mixture having a low content of aromatic halogen compounds is therefore desirable.
Methods for producing methylenediphenyl diisocyanate having a low content of chlorine compounds are known per se from the prior art.
DE-OS 2631168 describes the production of diisocyanates which can be adjusted with respect to their chlorine content. For this purpose a mixture of isomers comprising substantially 2,4′- and 4,4′-MDI is first freed in a distillation column from the majority of the impurities boiling higher than 4,4′-MDI and then the resultant distillate is freed by distillation from the impurities boiling more readily than 2,4′-MDI. The proposed technical solution, however, is very complex in terms of apparatus. The depletion in secondary carbamoyl chlorides in the resultant 4,4′-MDI is in addition frequently inadequate.
DE-OS 2933601 describes a method for producing polymeric MDI and monomeric MDI having a lower fraction of uretdiones and hydrolyzable chlorine compounds. In a first stage, binuclear MDI is separated off from PMDI (thin-film evaporator 175-210° C.). The distillate from the thin-film evaporator is condensed in the presence of an inert gas and then the MDI isomers are separated from one another by distillation. The resultant 4,4′-MDI, however, still comprises unwanted compounds that boil higher than 4,4′-MDI. In addition, the method does not always allow itself to be integrated in an economic manner in an overall process.
DD-P 288599 A5 describes a method for reducing the content of chlorine-comprising compounds in isocyanates by admixture with carbodiimides and subsequent stripping. The thermal dehalogenation, however, does not lead to complete breakdown of the halogen compounds. Thus the secondary carbamoyl chlorides may be incompletely removed. Owing to the high thermal stress of the resultant product, in addition, unwanted breakdown products form. The addition of carbodiimides, in addition to the stated reduction in chlorine, causes an increase in the molecular weight due to trimerization reactions.
Certain aromatic halogen compounds that are predominantly difficult to hydrolyze and have a higher boiling point than 4,4-MDI, however, may not be removed from 4,4′-MDI-comprising mixtures by methods of the prior art, or may not be removed to a sufficient extent. Furthermore, the methods known from the prior art may not always be integrated to a satisfactory extent into known methods for producing MDI.
It was therefore an object of the invention to find a method for purifying 4,4′-MDI-comprising mixtures which does not have the abovementioned disadvantages, or has them to a decreased extent.
It was an object of the present invention, in particular, to produce mixtures of MDI isomers, in particular mixtures of 2,4′- and 4,4′-MDI and also pure 4,4′-MDI having a lower content of uretdiones and uretonimines and also hydrolyzable chlorine compounds. The method should be able to be implemented with a low expenditure on apparatus and be mild toward MDI.
The object was, in particular, to provide a mixture comprising 4,4′-MDI, which mixture has a low content of hydrolyzable chlorine compounds. In particular, the content of chlorinated phenyl isocyanates and of chlorinated byproducts which pass into the phosgenation from byproducts of the aniline-formaldehyde condensation that are not separated off should be as low as possible.
The method should be able to be integrated with the lowest possible expenditure on resources into existing technologies for producing binuclear MDI.